Several types of drive units for crude oil feed pumps are known. For example, the pump rods of the so-called horsehead pump is driven by an electric motor through a gear mechanism including a counterweight. This drive results in equal speeds for the upstroke and downstroke. An energy recovery is not possible. To perform a slow downstroke and a faster upstroke, it is further known to use a cylinder pump as drive means, wherein the delivery flow of a hydraulic pump is supplied to the piston-rod-sided chamber of the cylinder. For performing the downstroke, the liquid volume displaced by the piston of the pump due to gravity is returned through an adjustable throttle to the reservoir. Again, an energy recovery is not possible. A drive means of this type is acknowledged in the preamble of claim 1.
A still further prior drive unit for a feed pump comprises a gas accumulator mounted atop of the pumping cylinder. In particular, the piston of the pumping cylinder is provided with a second piston rod and a second piston which limits the volume of the accumulator. When the piston of the pump is in the upper dead center, the gas pressure in the accumulator cylinder is such that the weight of the pump rods is at least counterbalanced. Both cylinder chambers of the pump are connected to an axial piston pump comprising a swash plate which can be swivelled over center. To lower the piston, the axial piston pump is swivelled in the one direction to supply liquid to the upper cylinder space of the pumping cylinder, whereby the piston is displaced downwardly, simultaneously compressing the gas pad in the accumulator. Accordingly, to lower the piston, energy must be supplied. At the lower dead center, the pressure in the accumulator has reached a maximum pressure and the axial piston pump is swivelled through the zero position into the second adjusting direction, whereby the flow direction is reversed and the lower cylinder space of the pumping cylinder is supplied with liquid to lift the pumping piston.
The upstroke motion is supported by the pressure in the gas accumulator such that the pressure difference between the accumulator pressure and the upstroke pressure must be balanced for by the pump. This arrangement thus makes possible an energy recovery.